At times copper-bearing minerals also contain small amounts of molybdenum, which dissolves into aqueous solution in copper mineral leaching conditions. In the copper production process, after leaching the solution purification stage often used is liquid-liquid extraction by means of organic extraction solution, in which copper is transferred to the organic phase, leaving most of the impurities in the aqueous solution. The extraction reagents used nowadays in the organic extraction solution of the liquid-liquid extraction of copper are generally different types of hydroxy oximes, such as for example 5-dodecyl salicylaldoximes. The use of hydroxy oximes also for the extraction of molybdenum is mentioned for instance in U.S. Pat. No. 3,415,616. It is also known that molybdenum is extracted with copper into the organic phase in copper extraction conditions. In the pH range of the extraction stages in copper extraction, molybdenum appears mostly as molybdic acid H2MoO4 and molybdate cations MoO22+. Kinetically, the extraction of molybdenum to the organic phase is rapid.
Earlier it has been observed that in connection with copper, molybdenum can also be extracted with a quinoline-based extraction reagent, as described for instance in Canadian patent publication 1061574. However, quinoline-based extraction reagents are not in general use nowadays in copper extraction processes.
It is also known in the prior art that molybdenum is not removed from organic solution in copper stripping conditions. As a result, molybdenum is gradually concentrated in the organic phase and consequently reduces the copper extraction capacity of the organic phase and lengthens the settling times of the phases in the settler. At high concentrations, molybdenum may also raise the viscosity of the extraction solution. Raised organic phase viscosity is one of the worst problems at the extraction plant, since in this case the organic extraction solution is firstly hard to pump and therefore demands more energy to form a dispersion. Secondly, raised viscosity generally means that the extraction reactions also slow down and thus the copper extraction capacity deteriorates.
U.S. Pat. No. 4,026,988 describes a method for the selective separation of molybdenum from an aqueous solution that also contains copper. According to the method, the extraction solution used is a molybdenum-selective α-hydroxy oxime, to which nonyl phenol is added to facilitate stripping. When the pH of the aqueous solution is kept in a range below 2, extraction of copper is fairly minimal, but molybdenum is extracted well. Copper extraction from an aqueous solution from which molybdenum has been removed, nevertheless requires in this case a second extraction process, so as a whole the method is not simple.
A method is described in Spanish patent publication 2156504 for separating copper and molybdenum from an aqueous solution by means of extraction. The extraction reagents used are commercial oximes in conditions in which both copper and molybdenum are extracted into an organic solution. Stripping takes place in two stages. In the first stage the organic solution is brought into contact with an aqueous sulphuric acid-containing solution, whereupon the majority of the copper is transferred to the aqueous solution, but in practice no molybdenum is transferred at all. After this, the second stage of stripping is performed, in which the organic solution is contacted with an ammonia-containing aqueous solution with a residence time of 10-60 minutes. When the ammonia concentration is 1 mol/l according to example 3, almost all the molybdenum is transferred to the aqueous solution. After stripping, the organic solution is scrubbed possibly with an acid-containing aqueous solution to remove the residual ammonia and is routed back into circulation.
In the methods described in both U.S. Pat. No. 4,026,988 and ES publication 2156504, the stripping of molybdenum requires its own equipment unit. In the situation where the quantity of molybdenum is small and the purpose is mainly to remove it from organic solution, separate stripping equipment constitutes a fairly substantial cost item. When an ammonia-based aqueous solution is used to strip molybdenum, an additional separate scrubbing stage is needed to remove the ammonia from the organic extraction solution.
WO application publication 2005/120677 describes an apparatus in which the organic extraction solution containing valuable metals is scrubbed of aqueous solution droplets in the organic extraction solution storage tank. The scrubbing solution, which is an aqueous solution, is mostly fed into the organic solution before this solution is routed into the tank. The scrubbing apparatus includes droplet coalescer equipment, and the remaining part of the aqueous solution is routed to the tank at the point of the droplet coalescer equipment. The organic solution is fed into the bottom part of the storage tank in several different substreams and removed from the surface section of the liquid layer at the rear end of the tank in several substreams. The scrubbing solution is removed from the bottom section of the rear end of the tank in several substreams.